In recent years, membrane technology has been extensively investigated for water treatment. In addition to removing particulates, some membrane processes that use membranes with very small pores are able to remove a significant amount of natural organic matter (NOM); however, membranes capable of removing NOM are also more easily fouled by NOM than are membranes that have larger pores.
In conventional membrane systems, molecules that are larger than the pore size of the membrane and those that adsorb to the membrane surface are retained, while smaller molecules pass through the membrane along with the permeate flow. Typical NOM removal efficiencies for membranes having pore sizes ranging from about 0.001-0.1 microns, commonly known as ultrafiltration membranes, are around 15-20%. It has been proposed that powdered activated carbon be used in conjunction with membranes to adsorb a portion of the NOM and allow the membrane to reject the larger particulate adsorbents. While the NOM removal efficiency of such a combined process can be quite high, several problems are encountered when one attempts to regenerate the activated carbon particles. Typically, in order to regenerate powdered activated carbon, the particles must be heated to high temperatures to burn off the NOM. The cost of generating such high temperatures has a negative impact on the economics of the combined process using powered activated carbon. In addition, when powdered activated carbon particles are heated to such high temperatures, a certain portion of the particles are consumed by combustion. If the particles are not regenerated, they must be disposed of, which results in attendant disposal costs.
In addition to powdered activated carbon, it has been proposed that freshly precipitated iron or aluminum oxides be used in conjunction with membranes to reduce fouling of the membrane. While freshly precipitated iron oxide and aluminum oxide particles are commonly used to adsorb NOM in water treatment processes that do not employ membranes, when used in conjunction with membranes, it is reported that the freshly precipitated particles themselves contribute to the fouling of the membrane.
With the ever increasing concern about the quality of drinking water, there continues to be a need for improved systems for effectively and economically removing contaminants such as natural organic matter from water. Such systems should provide effective removal of contaminants from the water and should be capable of being repeatedly used and regenerated in order to provide an economical alternative to existing water treatment processes.